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  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
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<body>
  <script>
    //父类有加法的方法
    class Father {
      constructor(x,y){
        this.x = x ;
        this.y = y ;
      }
      sum(){
        console.log(this.x + this.y);
      }
    }
    //子类继承父类的加法方法 同时 扩展减法方法
    class Son extends Father {
      constructor(x,y) {
        //利用super 调用父类的构造函数
        //super 必须在子类this之前调用
        super(x,y);
        this.x = x;
        this.y = y;
      }
      subtract(){
        console.log(this.x - this.y);
      }
    }
    var son = new Son(5,3);
    son.subtract();
    son.sum();



  </script>
</body>
</html>